Mu-opioid receptor (MOR) agonists such as morphine, fentanyl and remifentanil (remi) are the most effective perioperative analgesics for both acute severe postoperative pain and chronic severe pain states. The most serious, life-threatening side effect, which limits their dosage, is profound depression of breathing rate (bradypnea) and tidal volume. This risk is increased in the presence of other sedatives such as benzodiazepines (BZDs) and alcohol. Our studies indicate that neurons in the parabrachial/ K?lliker-Fuse nuclei (PB-KF complex) of the pons are very sensitive to low clinical opioid concentrations associated with bradypnea and have the potential to cause respiratory arrest. We have also discovered that neurons in a small parabrachial subregion (PBSR) control breathing frequency (fB) and appear to be the portal that mediates the opioid-induced bradypnea. Localized excitation of PBSR neurons increase fB, while those that decrease neuronal activity decrease fB even to the point of apnea. Thus, any drugs that affect the activity of PBSR neurons will have a major impact on breathing. Our working hypothesis is the PBSR functions as the major controller of fB by providing excitatory inputs to the rhythmogenic preB?tzinger Complex (preBC) that produces phasic inspiratory (I) and expiratory (E) neuronal discharge patterns. Depression of PBSR neuronal activity by systemically administered opioids alone or combined with sedatives leads to severe bradypnea or arrest. The PBSR also modulates the gain of the reflex control of I-duration (TI) and E-duration (TE) mediated by slowly adapting pulmonary stretch receptors (PSRs). To address the above hypotheses, the following specific aims will be pursued: 1) Precisely locate the PBSR in the dorsal pons near the PB-KF area that controls eupneic fB as suggested by preliminary findings, 2) Identify the PBSR neuron subtypes, determine if their axons project to the preBC/BC region and quantify their responses to PSR inputs, 3) determine how the discharge patterns of the various PBSR neuron subtypes are generated and controlled by A) NMDA and nonNMDA receptor mediated glutamatergic endogenous excitation and B) by GABAergic and glycinergic endogenous inhibition, 4) determine A) whether the GABAA receptors on neurons within this region are modulated by BZDs (e.g., midazolam), B) whether systemically-administered BZDs act on neurons within the PBSR (antagonized by microinjected flumazenil), and C) whether microinjected BZDs modulate the effects of iv remifentanil-induced bradypnea, 5) determine the role of GABAB receptors in the modulation of PBSR neurons and of TI and TE via microinjections of selective agonists and antagonists, 6) identify preBC neuron subtypes that mediate increases in fB evoked within the PBSR by AMPA stimulation and highly localized electrical stimuli, and 7 A) characterize the modulation of the PSR reflex control of TI and TE mediated by the PBSR, and B) and determine whether this modulation is due to PBSR inputs to the nucleus of the solitary tract (NTS) that alter the PSR neurotransmission to the second order neurons. Systemic i.v. infusions of ultra short-acting remifentanil will be used to produce bradypnea in an in vivo decerebrate canine model. Phrenic nerve activity will be recorded to measure TI and TE. Multibarrel micropipettes and a 16-electrode probe (NeuroNexus) will be used to simultaneously record the discharge of PBSR neurons while picoejecting neuroactive agents. Responses to PSR inputs and axon projections to the preBC region will be used to further classify the PBSR neurons. The PBSR appears to act as the portal through which breathing rate is controlled. Thus, the study of the neurophysiological and neuropharmacological characteristics of this discrete region will provide major insight into the mechanisms by which drugs adversely affect breathing frequency and suggest therapeutic measures to alleviate undesirable side effects. These studies will also provide important new information on the functional roles of PBSR neurons and the contribution of specific neurotransmitters/modulators to the discharge patterns of PBSR neuron subtypes in vivo and new insights into control of breathing mechanisms.